How to make parabolic particle form?

I've done very little scripting with Particles (beyond turning them on an off), so this was an interesting question for me. Here is a script that uses a Mathf.Sin() to create a wave as you've outlined.

 var magnitude = 1.0;
 var period     = 2.0;
 var maxParticles = 1000;
 
 private var particles : ParticleSystem.Particle[];
 private var ps : ParticleSystem;
 
 function Start () {
     ps = particleSystem;
     particles = new ParticleSystem.Particle[maxParticles];
 }
 
 function Update() {
     if (ps.particleCount > particles.Length) {
         particles = new ParticleSystem.Particle[ ps.particleCount];
     }
     
      ps.GetParticles(particles);
     for(var i = 0; i < ps.particleCount; i++) {
          var dist = particles[i].position.x - transform.position.x;
          particles[i].position.y = Mathf.Sin((dist % period) / period * 2.0 * Mathf.PI) * magnitude;
     }  
        
     ps.SetParticles(particles, ps.particleCount);
 }

Apparently you cannot get the maximum number of particles in script, so I use a variable plus some code that will resize the array if the variable is wrong.